1. Field of the Invention
The invention relates in general to a vertical cavity surface emitting laser (VCSEL). More particularly, the invention relates to growth methods and structures for distributed Bragg reflectors (DBRs) utilized in VCSELs.
2. General Background and State of the Art
Semiconductor lasers are widely used in optical applications, in part because semiconductor fabrication techniques are relatively inexpensive and yield reliable, consistent results. Also, they are easily packaged into current microelectronics. A relatively new class of semiconductor lasers, vertical cavity surface emitting lasers (VCSELs), has been developed through the evolution of this technology. Unlike conventional edge emitting lasers that emit light in a direction parallel to the semiconductor substrates where the lasers are formed, VCSELs have optical cavities perpendicular to the substrate, and thus emit optical radiation in a direction perpendicular to the substrate. In addition to various performance and application-adaptable improvements created thereby, VCSELs simply require reduced complexity in their fabrication and testing, as compared to conventional edge emitting semiconductor lasers.
Vertical cavity surface emitting lasers (VCSELs) have been proven to be solutions for low-cost transmitters for high-speed data communications at 980 nm and 850 nm and have shown great potential for cost-effective telecommunication systems at longer wavelengths as well, such as 1.55 xcexcm and 1.3 xcexcm. These long wavelength VCSELs will satisfy increasing demand for high speed data transmission over tens of kilometers. 10-Gigabit Ethernet is one example, which requires inexpensive transmitters with a data rate of 10 G bit per second (Gbps) and up to 40 km reach over single-mode fiber.
VCSELs are semiconductor lasers having a semiconductor layer of optically active material, such as gallium arsenide or indium gallium arsenide or the like, sandwiched between highly-reflective layers of metallic material, dielectric material, epitaxially-grown semiconductor dielectric material or combinations thereof, most frequently in stacks. These stacks are known as distributed Bragg reflectors, or DBRs. DBRs are used to reflect emitted light back into the active material of a VCSEL. As is conventional, one of the mirror stacks is partially reflective so as to pass a portion of the coherent light built up in the resonating cavity formed by the mirror stack/active layer sandwich. Laser structures require optical confinement and carrier confinement to achieve efficient conversion of pumping electrons to stimulated photons (a semiconductor may lase if it achieves population inversion in the energy bands of the active material.)
The development of vertical-cavity surface-emitting lasers (VCSELs) at the telecommunications-important wavelength of 1.55 xcexcm has been hindered by the absence of a substrate that is suitable for both technologically-developed distributed Bragg reflectors (DBRs) and quantum well active regions. In fact, despite the demonstration of VCSELs grown on a single substrate, the best results have been obtained through the fusion of InP-based active regions and AlGaAs-based DBRs.
To overcome mirror limitations on InP, several groups have examined AlGaAsSb-based DBRs, which have a refractive index contrast that is similar to AlGaAs-based DBRs at this wavelength. The high index contrast leads to a lower penetration depth than traditional InGaAsP-based DBRs and, therefore, implies lower optical loss in the structure. Only optically-pumped VCSELs using such DBRs, however, have been demonstrated.
While both short wavelength and long wavelength VCSELs have proven to offer excellent solutions for many applications in the evolving optical applications marketplace, they also have certain limitations and drawbacks that are well known in the art. Some of these drawbacks are inherent to the conventional materials used in the fabrication of Bragg mirrors for VCSELs grown on InP substrates. For example, the accuracy and reproducibility of an As, Sb composition in a AlGaAsSb semiconductor system is very difficult to achieve in DBR fabrication. While such materials have conventionally been considered and used as the best selection for the mirrors, they do not effectively optimize high reflectivity, good electrical conduction and low thermal resistance.
The present invention provides a method of growing a distributed Bragg reflector for use in a VCSEL using molecular beam epitaxy. The present invention also provides a method of fabricating a distributed Bragg reflector (DBR) in which the amount of particular semiconductor materials used in the DBR are controlled in the molecular beam epitaxy process.
The present invention provides a DBR for use in vertical cavity surfave emitting laser (VCSEL) having an Sb-based semiconductor material. The present invention further provides a method of enhancing thermal properties in a DBR by growing AlGaAsSb layers and InP layers on a substrate to form the DBR. A method of fabricating DBRs using these two types of layers is also provided.
One aspect of the present invention provides a method for controlling material composition in a distributed Bragg reflector. The method includes applying reflection high-energy electron diffraction (RHEED) oscillations in molecular beam epitaxy to a substrate, measuring the intensity of antinomy (Sb) atoms present in the substrate in response to the RHEED oscillations, and calibrating the amount of Sb to be incorporated into the susbtrate, with the amount depending upon the frequency of the RHEED oscillations induced by the Sb atoms.
Accordingly, one object of the present invention is to provide a method of controlling material composition in a distributed Bragg reflector for use in a VCSEL. It is another object of the present invention to provide a method of fabricating a distributed Bragg reflect for use in a VCSEL incorporating the above method of controlling material composition. It is still another object of the present invention to provide a VCSEL and method of fabrication of a VCSEL which enhances the thermal and reflective characteristics of the VCSEL.